tests/shell: bind mount private /var/run/netns in test container

tests/shell: bind mount private /var/run/netns in test container

Some tests want to run `ip netns add`, which requires write permissions
to /var/run/netns. Also, /var/run/netns would be a systemwide mount
path, and shared between the tests. We would want to isolate that.

Fix that by bind mount a tmpfs inside the test wrapper, if we appear to
have a private mount namespace.

Fixes

  $ ./tests/shell/run-tests.sh -- tests/shell/testcases/netns/0001nft-f_0

Optimally, `ip netns add` would allow to specify a private
location for those bind mounts.

It seems that iproute2 is build with /var/run/netns, instead the more
common /run/netns. Hence, handle /var/run instead of /run.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>